Conventionally, alumina (Al.sub.2 O.sub.3) is used as a dielectric material for microelectronic packages. It has excellent electrical (insulating), thermal and mechanical (especially strength) properties. Alumina based packages, generally containing 4-10 wt. % glass, require sintering temperatures above 1500.degree. C. which necessitates the use of refractory metals such as molybdenum or tungsten for the electrical interconnections so that the metal can be co-fired with the package. These metals have poor electrical conductivity as compared to highly conductive metals such as copper, and secondly, they require the use of strongly reducing atmospheres during co-firing necessitating expensive furnace systems.
The development of multilayer ceramic circuit boards is toward higher frequency, higher density and higher speed devices. Al.sub.2 O.sub.3 has a relatively high dielectric constant of about 9.9, causing high signal propagation delay and low signal-to-noise ratio (crosstalk). The signal propagation delay (t) in ceramic substrates is affected by the effective dielectric constant of the substrate (k') in the following equation: EQU t=(k').sup.0.5 /C
where C is the speed of light. It can be found that the signal propagation delay can be dramatically reduced by a reduction in the effective dielectric constant of the substrate. For example, if the dielectric constant of a material is reduced from 10 (approximately the k' of Al.sub.2 O.sub.3) to 5, the signal propagation delay can be reduced by 30%. A small signal delay is especially important for the substrate housing a chip with a very dense integrated circuit, for instance, very large scale integrated circuit (VLSI).
Furthermore, alumina has a coefficient of thermal expansion of about 7.4.times.10.sup.-6 /.degree.C. (in the 20.degree.-200.degree. C. range) as compared with 3.4.times.10.sup.-6 /.degree.C. for silicon. This mismatch in thermal expansion results in design constraints and reliability concerns when attaching a silicon wafer to the substrate.
Heretofore, most of the dielectric materials used in multilayer circuits have been conventional thick film compositions. A typical circuit is constructed by sequentially printing, drying and firing functional thick film layers atop a ceramic substrate which is usually 92-96 wt. % Al.sub.2 O.sub.3. The multiple steps required make this technology process intensive with the large number of process steps and yield losses contributing to high costs. Thick film technology nevertheless fills an important need in microelectronics and will continue to do so in the foreseeable future.
Recently, dielectric thick film compositions with low dielectric constants of 5 have been introduced. However, ceramic substrates with low dielectric constants less than 5.5 and thermal expansion coefficients which closely match that of silicon (3.4 ppm/.degree.C.) are not readily available.
Low temperature co-fired (LTCF) technology has been recently introduced as a method for fabricating multilayer circuits. This technology offers the combination of the processing advantages of high temperature co-fired (HTCF) technology and the materials advantages of thick film technology. These LTCF tape systems have firing temperatures below 1000.degree. C. and allow the use of high conductivity metals such as silver, gold, silver/palladium and copper (copper, however, requires reducing atmospheres). Most of these tape systems have dielectric constants between 6 and 8 and encompass a range of thermal coefficient of expansion (TCE).
Currently, there is no readily available low temperature co-fired dielectric tape system using a glass plus ceramic approach that offers both low dielectric constant (less than 5.5) and a TCE matched to silicon (3.4 ppm/.degree.C.).